Such screw machines are used mainly as double screw extrusion presses or double-screw injection molding or casting machines. Such a wear insert which is known from U.S. Pat. No. 3,268,949 comprises two bushes shaped as sectors of a cylindrical ring and two clamping bars forming the saddles which are tightened in corresponding longitudinal grooves in the screw housing in such a manner that the bushes are pressed against the interior walls of the cavity in the screw housing simultaneously. The cost of manufacturing this multi-part wear insert and the production of the corresponding internal shape of the cavity in the screw housing is quite high. Moreover, there remains between the bars forming the saddle, and the bushes, several gaps through which corrosive gases or liquid contained in the material to be processed can penetrate leading to damage to the interior wall of the cavity in the screw housing. In this way, the surface contact, particularly in the region of the saddle, between the wear insert and the screw housing is reduced which leads in time to poor and non-uniform heat transmission between the insert and the screw housing and thus to distortion of the wear insert. Futhermore, the bushes are initially distorted out of the arcuate cross-section by the clamping forces applied thereto.
It is already known to form the cavity in the screw housing with a larger cross-section than which corresponds to the external cross-section of the wearing insert and to fill this residual space with a castable material, the melting point of which is lower than the melting point of the material of the wearing insert and than the melting point of the housing material. A good heat transfer from the wearing sleeve to the screw housing and a largely distortion-free seat is in fact achieved by this means but the manufacture is complicated and costly. Furthermore the work outlay required to exchange a worn wearing insert is high.
It is also known to manufacture a wearing insert from two tubes welded together at their chamfered joint surfaces, while these tubes have externally along the joints, notchings which alternate with flattened surfaces, and while the welds connecting the tubes are arranged solely in the region of the notchings. It is intended to achieve by this means that very close tolerances can be observed during the manufacture of the wearing insert, so that the joint surfaces are in such snug mutual contact that so far as possible no material processed in the screw machine, such as, e.g., plastics, penetrates between the joint surfaces. However, such joint surfaces have the disadvantage that they corrode due to penetrating aggressive gas or condensate from the plastics melt. In a particularly disadvantageous manner, these joint surfaces simultaneously constitute the most heavily stressed point, namely the saddle point of the wearing insert, and therefore tend readily to fracture. Futhermore, in this case, firstly complicated manufacture of the wearing insert and secondly complicated exchange of such a wearing insert are obtained. Futhermore, here again a full surface contact of the wearing insert in the cavity of the screw housing is not ensured. A wearing insert of practically identical construction is known from U.S. Pat. No. 3,010,151, to which the above remarks apply.